The present invention pertains to composite materials of ceramic fibers within in an aluminum matrix. Such materials are well-suited for various applications in which high strength, low weight materials are required.
Continuous fiber reinforced aluminum matrix composites (CF-AMCs) offer exceptional specific properties when compared to conventional alloys and to particulate metal matrix composites. The longitudinal stiffness of such composite materials is typically three times that of conventional alloys, and the specific strength of such composites is typically twice that of high-strength steel or aluminum alloys. Furthermore, for many applications, CF-AMCs are particularly attractive when compared to graphite-polymer composites due to their more moderate anisotropy in properties, particularly their high strength in directions different that those of the fiber axes. Additionally, CF-AMCs offer substantial improvements in allowable service temperature ranges and do not suffer from environmental problems typically encountered by polymeric matrix composites. Such problems include delamination and degradation in hot and humid environments, particularly when exposed to ultraviolet (UV) radiation.
Despite their numerous advantages, known CF-AMCs suffer rawbacks which have hampered their use in many engineering applications. CF-AMCs generally feature high modulus or high strength, but seldom combine both properties. This feature is taught in Table V of R. B. Bhagat, xe2x80x9cCasting Fiber-Reinforced Metal Matrix Compositesxe2x80x9d, in Metal Matrix Composites: Processing and Interfaces, R. K. Everett and R. J. Arsenault Eds., Academic Press, 1991, pp. 43-82. In that reference, properties listed for cast CF-AMC only combine a strength in excess of 1 GPa with a modulus in excess of 160 GPa in high-strength carbon-reinforced aluminum, a composite which suffers from low transverse strength, low compressive strength, and poor corrosion resistance. At the present time, the most satisfactory approach for producing CF-AMCs in which high strength in all directions is combined with a high modulus in all directions is with fibers produced by chemical vapor deposition. The resulting fibers, typically boron, are very expensive, too large to be wound into preforms having a small-radius of curvature, and chemically reactive in molten aluminum. Each of these factors significantly reduces the processability and commercial desirability of the fiber.
Furthermore, composites such as aluminum oxide (alumina) fibers in aluminum alloy matrices suffer from additional drawbacks during their manufacture. In particular, during the production of such composite materials, it has been found to be difficult to cause the matrix material to completely infiltrate fiber bundles. Also, many composite metal materials known in the art suffer from insufficient long-term stability as a result of chemical interactions which can take place between the fibers and the surrounding matrix, resulting in fiber degradation over time. In still other instances, it has been found to be difficult to cause the matrix metal to completely wet the fibers. Although attempts have been made to overcome these problems (notably, providing the fibers with chemical coatings to increase wetability and limit chemical degradation, and using pressure differentials to assist matrix infiltration) such attempts have met with only limited success. For example, the resulting matrices have, in some instances, been shown to have decreased physical characteristics. Furthermore, fiber coating methods typically require the addition of several complicated process steps during the manufacturing process.
In view of the above, a need exists for ceramic fiber metal composite materials that offer improved strength and weight characteristics, are free of long term degradation, and which may be produced using a minimum of process steps.
The present invention relates to continuous fiber aluminum matrix composites having wide industrial applicability. Embodiments of the present invention pertain to continuous fiber aluminum matrix composites having continuous high-strength, high-stiffness fibers contained within a matrix material wherein there are substantially no phases at a fiber/matrix interface that enhance the brittleness of the composite (i.e., the composite is substantially free of brittle intermetallic compounds or phases, or segregated domains of contaminant material at the matrix/fiber interface that enhance the brittleness of the composite). The matrix material is selected to have a relatively low yield strength whereas the fibers are selected to have a relatively high tensile strength. Furthermore, the materials are selected such that the fibers are relatively chemically inert both in the molten and solid phases of the matrix.
Certain embodiments of the present invention relate to composite materials having continuous tows of polycrystalline xcex1-Al2O3 fibers having an average tensile strength of about 2.8 GPa contained within a matrix of substantially pure elemental aluminum having a yield strength of not greater than about 20 MPa or an alloy of elemental aluminum containing up to about 2% by weight copper (based on the total weight of the matrix) having a yield strength of not greater than about 90 MPa. Such composite structures offer high strength and low weight, while at the same time avoid the potential for long term degradation. Such composites may also be made without the need for many of the process steps associated with prior art composite materials.
A preferred article according to the present invention comprises a composite material comprising a plurality (e.g., a tow(s)) of continuous polycrystalline xcex1-Al2O3 fibers within a matrix, wherein the polycrystalline xcex1-Al2O3 fibers have an average tensile strength of at least about 2.8 GPa, wherein the matrix is selected from the group consisting of substantially pure elemental aluminum and an alloy of substantially pure elemental aluminum and up to about 2% by weight copper, based on the total weight of the matrix. Preferably, the composite material has an average tensile strength of greater than 1.17 GPa (170 ksi) (or even at least 1.38 GPa (200 ksi), or at least 1.72 GPa (250 ksi)).
In one embodiment, the continuous fiber aluminum matrix composites of the present invention are formed into wires exhibiting desirable strength-to-weight characteristics and high electrical conductivity. Such wires are well-suited for use as core materials in high voltage power transmission (HVPT) cables, as they provide electrical and physical characteristics which offer improvements over HVPT cables known in the prior art.
Another preferred article according to the present invention comprises a composite material comprising a plurality (e.g., a tow(s)) of continuous polycrystalline xcex1-Al2O3 fibers within a matrix, wherein the matrix is an aluminum matrix that is substantially free of material phases or domains capable of enhancing brittleness of both the fibers and the matrix. Yet another preferred article according to the present invention comprises a plurality (e.g., a tow(s)) of continuous polycrystalline xcex1-Al2O3 fibers within matrix selected from the group consisting of a substantially pure elemental aluminum matrix and an alloy of substantially pure elemental aluminum and up to about 2% by weight copper.
In yet another aspect, the present invention provides a method of making an article, the method comprising:
providing a preform comprising continuous polycrystalline xcex1-Al2O3 fibers in a mold;
pressure infiltrating the mold with a metallic melt material selected from the group consisting of substantially pure elemental aluminum and an alloy of substantially pure elemental aluminum with up to 2% by weight copper into the mold such that the melt infiltrates the fibers; and
solidifying the infiltrated melt material and removing the resulting article from the mold to provide an aluminum matrix composite article comprising a composite material comprising a plurality of continuous polycrystalline xcex1-Al2O3 fibers within a matrix comprising aluminum, wherein the matrix is selected from the group consisting of substantially pure elemental aluminum and an alloy of substantially pure elemental aluminum and up to about 2% by weight copper, based on the total weight of the matrix.
In yet another aspect, the present invention provides a method of making an article, the method comprising:
providing a preform comprising continuous polycrystalline xcex1-Al2O3 fibers in a mold;
pressure infiltrating the mold with a metallic melt material selected from the group consisting of substantially pure elemental aluminum and an alloy of substantially pure elemental aluminum with up to 2% by weight copper into the mold such that the melt infiltrates the fibers; and
solidifying the infiltrated melt material and removing the resulting article from the mold to provide an aluminum matrix composite article comprising a composite material comprising a plurality of continuous polycrystalline xcex1-Al2O3 fibers within a matrix comprising aluminum, wherein the matrix is substantially free of material phases or domains capable of enhancing brittleness of both the fibers and the matrix.